This invention relates in general to the field of mineralogical exploration and mining analysis, including the practice of analyzing field samples to determine the presence of a selected substance, and, more particularly, to a field portable system using neutron activation for detecting a desired element.
The mineral exploration industry dedicates substantial resources to finding mineral deposits with concentrations of desired minerals that are large enough to allow economic extraction of desired minerals. To find such deposits, mining companies typically send individual technicians to field sites to take samples. Field sites are initially selected by indirect indications of deposits which may be obtained from geological indications such as visible host rock alteration or structural features. Technicians take rock and soil samples from each field site, label the samples according to their original location, and transport them to a laboratory for analysis. Laboratories use exact measurement techniques to determine the presence of desired elements in each sample. For instance, techniques such as mass spectrometry can precisely determine the chemical makeup of each sample. Based upon this laboratory analysis, a mining company may determine the feasibility of commercial extraction of the mineral resource. Often, however, additional testing of field sites having promising samples must occur to support the commercial exploitation decision.
Recent advances in the efficiency of extraction technologies allow economic extraction of certain precious metals from extremely low concentrations of mineral deposits. For example, the mineral exploration industry can now economically extract gold from subsurface deposits having gold concentrations of only tens of parts per million. These new technologies can also economically extract gold in open pit deposits in concentrations of less than one part per million. Thus, analysis of field samples should provide detection ranges for gold in the low parts per billion detection range. The industry may need similarly precise measurements for other elements.
Although laboratory analysis of field soil and rock samples provides accurate detection capability for trace concentrations of precious metals, laboratory analysis imposes a number of disadvantages. For instance, once a sample is taken from a field site, it may need to be transported long distances to an appropriate laboratory for analysis. En route or during transportation, the sample may be mislabeled or misidentified, leading to inaccurate results. From the time of shipping to the time at which results are available from the laboratory, several weeks may pass. Also, laboratory analysis typically includes input by third parties unrelated to the mineral exploration company, which can lead to security leaks when a concentration of a valuable element is discovered. In the time that it takes to obtain laboratory results, the mineral rights to a valuable property may be lost to commercial espionage.
In accordance with the present invention, a system and method is provided that substantially eliminates or reduces disadvantages and problems associated with previously developed methods for analyzing field rock and soil samples. The field sample is irradiated ex situ by exposure to a portable neutron source. The activated sample is then analyzed by a radiation detector to determine the spectrum of radiation it releases. The system compares the radiation spectrum against the spectrum that would occur with known concentrations of selected isotopes, allowing a determination of the concentration of the selected isotope in the sample.
More specifically, the present invention uses a portable neutron source, such as a moderator assembly or a subcritical reactor. Examples of potential neutron sources include californium 252, americium-beryllium, plutonium-beryllium, radium-beryllium. The moderator assembly is comprised of a moderator, a neutron source, an irradiation chamber. The subcritical reactor is comprised of the same elements with the addition of a fissile element such as uranium. The method includes placing a field sample in the irradiation chamber long enough to allow absorption of an adequate number of neutrons by nuclides of the desired element. The sample is then removed from the irradiation chamber and placed in a separate gamma ray detector, such as a high purity intrinsic germanium semiconductor detector, to allow detection of gamma ray radiation produced by the activated sample. The gamma ray detector is isolated from the neutron source to avoid interference from radiation produced by the neutron source. In one embodiment, the detector is placed in a detector shield to prevent interference from environmental gamma ray radiation. An analyzer, such as a multichannel analyzer and spectroscopy amplifier operating with a personal computer, provides the results of the gamma ray detector to a technician who can determine the presence and concentration of if selected elements in the field sample by determining the concentration of selected isotopes that decay and if produce gamma rays having predictable characteristics.
The present invention provides important technical advantages for mineral exploration. For instance, the present invention allows timely determinations of the concentration of trace elements in field samples by avoiding delays inherent to laboratory analysis, such as those caused by transport. Detection accuracy at field sites is adequate to allow determinations of the feasibility for commercial exploration of a given mineral resource. The accuracy can be easily adjusted to measure different concentrations of trace elements in accordance with the economic feasibility of mining each trace element. The detection of a number of elements is possible by varying the duration for which the sample is exposed to the neutron source and by measuring for different types of radiation. Secrecy of the results for each sample can be more easily maintained, and the results can be used to focus an investigation of a given field site at promising areas. Thus, mineral exploration companies can form, implement and modify exploration strategies and property purchase decisions in a timely manner as field test results become available. Additionally, the measurements provided by the present invention can enable differentiation among lithologic formations and can support geologic mapping for structural analysis, seismic risk studies, oil exploration and other purposes.